Modular grid understructure



FIG. 1 is a front view showing my new design;

FIG. 2 is a left side view thereof;

FIG. 3 is a right side view thereof;

FIG. 4 is a top view thereof;

FIG. 5 is a bottom view thereof; and,

FIG. 6 is a perspective view.

The broken lines showing floor tiles are for illustrative purposes onlyand form no part of the claimed design.

The ornamental design for a modular grid understructure, as shown anddescribed.